The invention relates to a computed tomography method for forming a scannogram by means of a computed tomography apparatus which includes a scanning unit for the acquisition of measured values that is rotatable about an axis of rotation and is provided with a radiation source and a detector unit, a relative motion which includes a displacement in the direction parallel to the axis of rotation taking place between the scanning unit and an examination zone during an acquisition of measured values, followed by the extraction of a scannogram from the measured values. The invention also relates to a computed tomography apparatus which is suitable for carrying out such a method, and to a computer program for controlling a computed tomography apparatus in conformity therewith.
It is known that a scannogram of an examination zone is formed in that a relative motion occurs between the examination zone and the scanning unit during the acquisition of the measured values, which relative motion includes merely a translation in the direction parallel to the axis of rotation. The measured values thus relate to line-shaped or strip-shaped sections of the examination zone which are combined so as to form a two-dimensional image or scannogram. The amount of calculation work required for such a scannogram is small in comparison with that required for the reconstruction of a computer tomogram. The dose required for forming a scannogram of adequate image quality is significantly smaller than the dose that would be required for the reconstruction of the attenuation in a section of the same length by means of the customary CT methods (CT=Computed Tomography). Therefore, scannograms are customarily used to define the region (shortened in comparison with the scannogram) which is of interest to the diagnosis and is to be imaged by means of a CT method.
Conventional scannograms, however, have limitations in that the direction wherefrom the examination zone is irradiated is predetermined as well as the projection geometry which is imposed by the distance between the radiation source and the detector unit. Therefore, during the formation of a scannogram it may occur that the region in the scannogram which is important to the diagnosis cannot be recognized, for example because it is masked by a bone structure. It is an object of the present invention to provide an essentially more flexible method of forming a scannogram.
This object is achieved by taking the following steps:
rotating the scanning unit about the axis of rotation during the acquisition of the measured values,
reconstructing from the measured values a 3D data set which is three-dimensionally dependent on the attenuation of the radiation in the examination zone covered by the scanning unit,
calculating at least one synthetic projection image from the 3D data set.
At the first glance it seems like a paradox to derive the measured values for a scannogram by making, like during the actual CT examination, the scanning unit rotate about the axis of rotation during the data acquisition and by reconstructing the attenuation in the examination zone by way of a comparatively intricate reconstruction method, thus yielding a 3D data set. These steps are also carried out during the actual CT examination, so that the question arises why the regions of interest for the diagnosis (ROI) for the subsequent CT method should be defined on the basis of a scannogram whose formation also requires the execution of a CT method, that is, even for a more extensive region in most cases. Moreover, it is also necessary to derive a synthetic projection image which serves as a scannogram from said 3D data set. The term xe2x80x9csyntheticxe2x80x9d is used for the projection image in this context because it is not formed directly like a scannogram but must be calculated from the previously reconstructed 3D data set.
The invention is based on the recognition of the fact that the 3D data set required for the formation of a scannogram can be acquired while using a dose which is not larger than that used for a conventional formation of a scannogram. This dose is significantly smaller than the dose required for the actual CT examination of a section of the same length, so that the attenuation in the individual voxels of the examination zone can be reconstructed with a very poor signal-to-noise ratio only. However, because the image value for each pixel in the scannogram is derived from a plurality of voxels of the examination zone, an adequate signal-to-noise ratio will be obtained for the scannogram derived from said data set. Consequently, the dose will not be higher than during a conventional formation of a scannogram.
The choice of the projection geometry is completely free. For example, the scannogram can be derived from the 3D data set by parallel projection for which voxels situated on parallel rays are taken into account for calculating the image value in the pixel in which the relevant ray terminates. Projection images can be formed with a projection direction that can be chosen at random, that is, also with a projection direction which is not perpendicular (oblique) relative to the axis of rotation. The projection geometry need be fixed only after the acquisition of the measured values for the 3D data set and a plurality of different scannograms can be derived from one 3D data set without it being necessary to acquire measured values again while exposing the patient to a further radiation dose.
A CT method must be suitable for the examination of heavy as well as thin patients. If the examination of all patients were performed with always the same radiation intensity and the same radiation quality, either an excessively high radiation dose would be applied in the case of a thin patient or an inadequate signal-to-noise ratio would be obtained in the event of a heavy patient. The version disclosed in claim 2 enables preselection of the radiation quality and/or radiation intensity for a subsequent CT examination in such a manner that there is no unnecessary radiation load and that an adequate signal-to-noise ratio is ensured.
Even when such an adaptation to the relevant patient has been performed, problems may arise due to the fact that in the case of lateral irradiation the attenuation is stronger than in the case of irradiation from the front (a.-p.), so that either the radiation load becomes too high for one irradiation direction or the signal-to-noise ratio becomes too poor for the other irradiation direction when a patient is examined with a constant radiation intensity and a constant radiation quality. Such problems can be avoided by means of the version of the invention as disclosed in claim 3; this version enables a given signal-to-noise ratio to be obtained with an as small as possible radiation load for all irradiation directions, i.e. for all positions of the scanning unit.
In such a computed tomography apparatus the measured values could in principle be acquired by making the scanning unit rotate in a given position so as to acquire the measured values for the reconstruction of a slice or layer, followed by displacement of the scanning unit in a direction of the axis of rotation so as to acquire the measured values for a neighboring slice, etc. The helical relative motion disclosed in the further version of claim 5, where rotation and displacement take place continuously, however, is more advantageous because it involves a smooth transition from one radiation source position to the next.
Claim 6 describes an attractive further embodiment of the computed tomography apparatus. Granted, the scannogram could also be formed by means of a computed tomography apparatus in which the radiation source emits a fan-shaped radiation beam (fan beam) and the detector unit comprises only a single line, but the embodiment according to claim 6 enables faster acquisition of the measured values.
Claim 7 describes the software for the control unit of the computed tomography apparatus whereby the method according to the invention can be carried out.